As semiconductor fabrication moves to the 25 nm node, ultrafine particles (those under 100 nm) generated by and on corona emitter points now pose a threat to manufacturing. Corona ionizers employ high voltages to emitter point(s) to create an ion current. Cleanroom corona ionizers can be very effective at controlling a wide range of static related issues in semiconductor fabrication, but corona ionizers create micron, sub-micron and deep sub-micron particlesi.

Signicant work has been done to study the mechanisms by which corona ionizers create particlesii. A variety of theories have been put forward on the subject. In summary, there are two dominant mechanisms, both related to the corona process itself. These processes are both a direct consequence of the energy density associated with the intense electric field required to create the ions. The emitter points of a corona ionizer must be quite sharp and the plasma created in the atmosphere immediately adjacent to the tip is characterized by a very high temperature.

In the first process, high temperature effects the physical integrity of the emitter point material and emitter material is sloughed offiii,iv. Emitter sloughing is currently reduced in cleanroom corona ionizers by optimization of the corona emitter point shape and material and applied voltages. These design subtleties lower the temperature of the plasma.

The second process involves the interaction of the corona plasma with airborne molecular contaminants in the cleanroom air. There are many solvents whose vapors become dissolved in the atmosphere and are referred to as AMCs. For example, hexamethyldisilazane (HMDS) is a large molecule used as a wetting agent in the photolithography process. It is well known that ionizers require much more frequent cleaning in a litho bay, but it is less well realized that additional particles are released from corona ionizer points into the cleanroom.

Any AMCs which pass through the corona region will react and agglomerate into small particles which are attracted to the emitter points by the dielectrophoretic force. When the particles contact the tip, some undergo charge exchange and are repelled into the cleanroom. Moving to a broader emitter tip made up of material which has good thermal conductivity lowers the temperature of the plasma and reduces the size of the agglomerated particles. In our experience, it makes the majority of them smaller than can be measured with a conventional laser particle counter. However, they still exist, and can be measured using a condensation nucleus counter. Today, these very small (<100 nm) particles are comparable to or larger than the feature size on modern semiconductors (CD~25 nm). Alpha ionizers utilize polonium 210 (Po210), a naturally occurring element in the soil, plants, vegetables, etc. The source emits 5.3 MeV alpha particles. The alphas move through the air colliding with air molecules as they go. Each time they collide, they give up some of their energy by ionizing the molecules they strike. On the average, each alpha particle travels 3 cm though the air and creates hundreds of thousands of ion pairs along its path before it stops.

At that point, the alpha particle picks up two electrons and becomes a helium atom, and drifts up through the atmosphere. There is no corona, no plasma and no agglomeration. There are no emitter tips to wear. There are no plasma regions and hence no agglomeration of AMCs. The process is inherently clean.

AlphaBoost™ bipolar ionization systems utilize clean, alpha ionization technology and extends the effective range by employing low intensity electric fields to move the ions rapidly away from the alpha source and into the laminar flow of the cleanroom. The nominal, variable pulsed voltage propel the ions and enables the alpha source to provide reliable protection at up to a meter in distance. AlphaBoost™ is designed to provide an ionizer which is safe, clean and requires NO maintenance for a full year.

Have you ever wondered what it means when a heel grounder described as having a “1 meg resistor”? or a 2 meg resistor? The size of a current limiting resistor is often included in the description of personal grounding products, but what does it mean and how concerned should I be with the size of the resistor when I am buying heel grounders? Today we will answer these questions but first some background:

What is a Resistor?

A common resistor

A resistor is a component of an electrical circuit that resists the flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current . Most grounding products have a current limiting resistor and it is most commonly one megohm, rated at least 1/4 watt with a working voltage rating of 250 volts. This ensures that the flowing current will be within safe levels.

Heel Grounders and ESD Protection?Heel grounders discharge static from a person to ground by connecting the person to a grounded walking surface. A heel grounder is made from a conductive material and is worn on each shoe. A conductive ribbon placed inside the wearer’s shoe or sock makes electrical contact with the skin through perspiration. The ribbon is joined to a resistor which limits current should accidental exposure to electricity that may occur. The other end of the resistor is joined to a conductive sole. This sole contacts a grounded ESD floor mat or ESD flooring system. Heel grounders should be worn on both feet to maintain ground contact while walking. UL and OSHA recommends a minimum of 1 megohm resistance to ground (RTG) in order to limit current for safety purposes.

Resistors and Heel Grounders Heel grounders come with either a 1 megohm or 2 megohm resistor. When one foot is on the ground, a 1 megohm heel grounder gives you an RTG of 1 megohm and a 2 megohm heel grounders results in 2 megohm RTG. But when both feet are on the ground, the sum of the resistors yield a RTG of 1/2 that measurement. For example:

When you wear two, 1 megohm heel grounders and have both feet on the floor, your RTG is only 1/2 megohm, NOT 1 megohm!

If you wear two, 2 megohm heel grounders an have both feet on the ground, your RTG is 1 megohm.

By wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.

This can be sometimes confusing. So we made a diagram to illustrate:

Conclusion

Heel grounders are an effective and popular way to ground mobile personnel in ESD areas. Heel grounders use built in resistors to limit any potential current that flows through the device to ensure operator safety. The size of the resistor that you use depends on the standards of the specific operation, but by wearing two heel grounders with a 2 megohm resistor you are complying with UL and OSHA at all times.